1. Field of the Invention
The present invention relates to a high-frequency power amplifier, and more particularly to an improvement in stability of a high-frequency power amplifier using a bipolar transistor.
2. Description of the Related Art
Recently, portable terminals such as portable phones have been spreading. When developing such recent models, a key point to consider is to reduce their size and weight, which requires a high-frequency power amplifier having a high output power capacity as a key component.
Heterojunction bipolar transistors (hereinafter abbreviated as xe2x80x9cHBTsxe2x80x9d) have been used for such high-power high-frequency power amplifiers in many portable phones since they have a large current gain xcex2 and, furthermore, types (GaAs types) of those having their emitter composed of AlGaAs and their base composed of GaAs provide high speed operation.
FIG. 5 is a circuit diagram of a conventional high-frequency power amplifier.
In FIG. 5, reference numeral 100 denotes a high-frequency power amplifier; 102 a substrate; 104 an MMIC chip; 106 an HBT for high-frequency power amplification; 108 an input matching circuit for the HBT 106; 110 an output matching circuit for the HBT 106; 112 a bias circuit for controlling the base potential applied to the HBT 106; 114 an isolation resistance for the bias circuit 112; 116 a signal input terminal for high-frequency power signals; 118 a signal output terminal for high-frequency power signals; 120 an output-control input terminal; 122 and 124 power source terminals for DC voltages; and 126 a ground terminal.
High-frequency signal power fed to the signal input terminal 116 is passed through the input matching circuit 108 and amplified by the HBT 106. The amplified high-frequency signal power is then output from the signal output terminal 118 through the output matching circuit 110. At that time, the base electrode of the HBT 106 is supplied with a base current at a predetermined voltage from the bias circuit 112 controlled by a control signal fed through the output-control input terminal 120. The isolation resistance 114 is provided between the bias circuit 112 and the base electrode of the HBT 106 so as to suppress part of the signal power from leaking to the bias circuit 112 (suppressing the sneak leakage power). When leakage power flows to the bias circuit 112, the bias circuit becomes unstable, reducing the stability of the power amplifier, and as a result, an excess current may flow due to occurrence of unwanted oscillation, which may cause a reduction in the reliability of the power amplifier.
As the output power of amplifiers becomes higher, signal power input to the HBT 106 is increased, making the leakage power greater. To suppress the increase in the leakage power, it is necessary to increase the resistance value R1 of the isolation resistance 114. On the other hand, the HBT 106 requires that a base current Ib supplied to its base electrode and the base potential V2 each must have a certain required value. However, increasing the resistance value R1 of the isolation resistance 114 to suppress the leakage power reduces the potential V2 applied to the base electrode as expressed by the formula
V2=V1xe2x88x92Ib*R1, 
where V1 is the output voltage of the bias circuit 112, and the base current Ib is fixed.
Therefore, in some cases, it is impossible to apply the base potential V2 of a required value.
To solve this problem, an inductor having a high-impedance characteristic in a target frequency band may be formed on the MMIC chip 104 in place of the isolation resistance 114 so that it is possible to reduce the amount of signal power leaking to the bias circuit 112 without reducing the potential V2 applied to the base of the HBT 106. However, this increases the size of the MMIC chip, thereby increasing its cost.
Incidentally, the inventions disclosed in Japanese Laid-Open Patent Publications Nos. 11-274867 (1999) and 10-322145 (1998) each use an FET as an amplifier transistor, and its gate bias resistor is also formed of an FET. However, the present invention relates to a bipolar transistor and therefore has a configuration different from those of the above-disclosed inventions.
On the other hand, Mitsubishi Denki Giho VO174 (No.6) issued 2000 describes a dual-band HBT MMIC amplifier on pages 43(401) through 46 (404), but does not mention the present invention described below.
The present invention has been made to solve the above-described problems. Therefore, it is an object of the present invention to provide a high-frequency power amplifier which is highly stable and inexpensive and in which signal leakage power to the bias circuit is small.
According to one aspect of the invention, there is provided a high-frequency power amplifier comprising: a semi-insulating semiconductor substrate; a first bipolar transistor having a base electrode for receiving a signal through an input matching circuit, and a collector electrode for outputting an amplified signal to an output matching circuit, the first bipolar transistor being disposed on the semiconductor substrate; a bias circuit for supplying a base current to the first bipolar transistor, the bias circuit being disposed on the semiconductor substrate; a first terminal connected to an output end of the bias circuit and disposed on the semiconductor substrate; a second terminal connected to the base electrode of the first bipolar transistor and disposed on the semiconductor substrate; a reactance element connected between the first terminal and the second terminal; and a mounting substrate having the reactance element and the semiconductor substrate disposed thereon.
Accordingly, the present invention can reduce the amount of the signal power leaking to the bias circuit without increasing the surface area of the semiconductor substrate, ensuring a stable operation of the bias circuit, and can provide an inexpensive and highly reliable high-frequency power amplifier.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.